Electronic – Driving LED with an N-Channel MOSFET

ledled-drivermosfet-driver

I am driving a red LED with a forward voltage of approx. 2V as shown in the picture. The driving signal comes from a microcontroller with a logic HIGH of 1.8V. The LED is being sourced from an independent supply of 3.3V. The circuit works but there is something in my measurements that I cannot explain.

I measured 3 mA through the LED branch. I need more current through the LED (about 10 mA would be ideal). Then I reduced R1 in steps until I had about 10 ohms. Yet the current in the LED did not change much. Sometimes I measured about 8 mA at the beginning of the measurement which after some seconds ended up in 4 mA.

  • Why is this happening? I also reduced R2 but it seems to have no effect either.

  • How could I increase the LED using a control signal of 1.8V and powering the LED with 3.3V?

LED driven by MOSFET

Best Answer

The problem is most likely that the mosfet does not have enough voltage to fully turn on. Vgs is probably not high enough. I couldn't find the datasheet for the CMPD7002 (or any part named that). First make sure the Vgs will give you enough current, find how much Vgs you need for the Rds on. You'll need to do one or more of these things:

1) If the GPIO's on the microcontroller are 3.3V tolerant (or if 3.3V is fine with a pull down) Then use open drain on the GPIO and a pull up to 3.3V

2) Use a circuit or buffer to translate 1.8 to 3.3V

3) Select a different mosfet that turns on at 1.8V and has a low rdson at 1.8V.

4) Calculate the current vs LED drop and size R1 correctly.